As is known, the part of the foot that has the greatest perspiration effect is the sole of the foot.
Therefore, the region of the shoe where moisture due to perspiration can accumulate the most is the interface between the sole of the foot and the sole of the shoe.
There, the sweat generated saturates the air with humidity and mostly condenses, stagnating on the insock.
Only a small amount of the moisture produced by sweating spreads to the sides of the upper and exits from them if they are vapor-permeable.
Such effect of sweat stagnation in the plantar region is particularly conspicuous in rubber-sole shoes; in fact, in these cases vapor permeation through the sole is prevented by its total impermeability.
As is known, the stagnation of sweat in the plantar region causes to the user of the shoe a feeling of discomfort and also constitutes a preferential site for the growth of bacterial cultures, which are known to cause bad odors.
Therefore, it is a commonly felt need to obviate the stagnation of moisture from sweating at the plantar region of shoes.
A first attempt aimed at meeting this need consists of the solution proposed in EP 0382904.
The teaching contained in such patent consists in dividing the rubber sole into two layers, the lower one of which has through micro-perforations, and in interposing a semipermeable membrane between the layers, in order to avoid water infiltrations and thus obtain a sole that is impermeable to water in the liquid state and permeable to water vapor.
For the sake of simplicity, hereinafter an element with the property of being impermeable to water in the liquid state and permeable to water vapor is referenced as waterproof and vapor-permeable.
The semipermeable membranes that EP 0382904 teaches to use are for example of the type disclosed in U.S. Pat. No. 4,187,390 and U.S. Pat. No. 4,194,041 by W. L. Gore or U.S. Pat. No. 6,228,477 by BHA Technologies.
Such membranes are provided by means of thin waterproof and vapor-permeable films made of expanded polytetrafluoroethylene, e-PTFE, with thicknesses that vary generally from 15 microns to 70 microns.
Their microstructure is characterized by the presence of dense areas, called nodes, interconnected by stretched filaments, called fibrils.
These semipermeable membranes, conceived initially for the military field, have been developed and used in the clothing and footwear field in order to avoid the accumulation of vapor from sweating in items of clothing and to provide shoes with uppers having waterproof and vapor-permeable linings.
Because the market of the clothing and footwear field has always demanded soft and comfortable items, in the described applications there is a strong need to ensure that the membrane, intended as a functional layer, does not compromise such characteristics.
This need has been expressed as a true technical preconception that has entailed the use of membranes provided with low thicknesses in order to be laminated with supporting and/or aesthetic finishing materials, such as fabric or leather, so as to obtain finished laminated elements that have high characteristics of flexibility, easy bending, softness, surface slipperiness, compressibility and extensibility and low weight per unit surface.
However, the films that provide such membranes have poor characteristics of mechanical strength, precisely because of their low thickness.
In fact, it should be noticed that the strength value of the laminated element derives mainly from the characteristics of the fabric layer or supporting layer to which the membrane is coupled.
In particular, the available films made of polymeric material used to provide said membranes, as mentioned, have thicknesses generally from 15 microns to 70 microns, which give them poor penetration resistance, i.e., lower than 5 N.
The expression “penetration resistance” designates the characteristic defined by a measurement performed according to the method illustrated in the ISO 20344-2004 standard, chapter 5.8.2 “Determination of the penetration resistance of the sole”, regarding safety footwear.
Such poor mechanical penetration resistance has brought the inventor of EP 0382904 to prevent contact of the membrane with foreign objects by limiting the diameter of the sole holes that the membrane faces.
This solution, however, has been found to limit the area of the sole assigned to vapor permeation and has also made it easy for the holes to become obstructed, a fact which compromises considerably the vapor permeation efficiency of the sole.
A further solution is proposed in U.S. Pat. No. 6,508,015 by Max Rauch, in which a structure with two superimposed layers is proposed, the upper one, i.e., the one designed to be directed toward the upper part of the shoe, is elastic and permeable to water vapor.
The lower layer, which covers less than 70% of the upper layer, has a supporting function and provides the tread.
The upper layer is made for example of sintered plastics or non-woven fabrics so as to present a microporous structure that in any case is not waterproof.
A drawback of this solution consists in that during use of the shoe, the microporous upper layer, left widely exposed by the lower layer, tends to become impregnated with water when it comes into contact with it, absorbing it, retaining it and partly releasing it over time, dirtying the surfaces on which one walks.
Moreover, the upper layer tends to degrade in contact with the absorbed water.
In order to obtain the waterproofness of the sole, this patent teaches to treat the upper layer so as to render it water-repellent or to cover it with a waterproof and vapor-permeable membrane arranged above it.
However, the hydrophobic treatment is of limited duration, causing the sole to lose its waterproofness.
Besides, the vapor-permeable and waterproof membrane arranged to cover the upper layer is exposed to the action of the insole, which during walking thereon may slip, applying an abrasive action that tends to damage it.
This damage is facilitated by the cyclic flexing and traction stresses to which the membrane is subjected during use of the shoe, which flexes in following the motion of the foot that wears it.
Moreover, the easy accessibility of the membrane by condensed sweat exposes it to the action of residues, such as mineral salts and fatty substances, that may deposit thereon during the evaporation and permeation of the sweat.
Such residues tend to obstruct the pores of the membrane, compromising its permeation efficiency.